Stevia is a high potency sweetener more than 200 times as sweet as sugar and is obtained by hot water extraction of Stevia rebaudiana Bertoni which belongs to family Compositae. Rebaudioside A is known to have less bitter taste and the most similar sweetening quality to sugar among the sweetening ingredients of stevia extracts and is 400 times sweeter than sugar, and occupies about 20% or so in the extracts in the case of non-modified plants. The most predominant ingredient in the extracts is stevioside which is a precursor to rebaudioside A. In order to produce high purity rebaudioside A, it is necessary to perform improvement of seeds with high content of rebaudioside A, cultivation, harvest, breed management, and the like, which entail economic problems in terms of time and cost.
In order to solve these problems, research has been carried out to convert stevioside into rebaudioside A. Typical examples include a method for converting stevioside into rebaudioside A by binding a glucose molecule to stevioside through enzymatic conversion using beta-1,3-glucanase derived from soil microorganisms (Korean Patent Publication 2004-0026747A and U.S. Pat. No. 6,469,947). Typical examples of sugar donor substrates employed in enzymatic conversion may include curdlan (beta-1,3-glucan). Curdlan is a high cost material with low solubility and has a drawback in that industrial application thereof is difficult. Furthermore, although a report says that curdlan converts stevioside into rebaudioside glycoside through fungal fermentation, the cultivation of the microorganisms takes about 15 days and also produces other steviol glycosides, such as rebaudioside B and the like, which make the conversion rate to final rebaudioside A only around 40%.